Hi @ababab ,
The fall may have loosened the video cable connection on the motherboard or worse damaged the LCD screen. Screens cannot be repaired, they have to be replaced.
Here’s a link to the ifixit [guide|36969] guide. Use the guide to inspect the video cable connection or to replace the screen assembly.
Screen assemblies are available online. Just search for ''Nexus 7 2nd Gen LCD screen assembly'' to get results.
Here’s a link to just one [https://www.amazon.com/Google-Nexus-2nd-Gen-2013/dp/B00GZBH312|supplier] of the part. It is shown only to give you an idea of the cost of the part.
Hopefully this is of some help.